Boreal forests are usually associated with the coniferous trees which predominate in them. However, if they were to be typified by their most widely-distributed tree, then it would be aspen which best characterises these northern forests. European aspen (Populus tremula) is one of the most widely distributed trees in the world, occurring from the Arctic Circle in Scandinavia to North Africa, and from Britain across most of Europe and north Asia to China and Japan, while quaking or trembling aspen (Populus tremuloides) is the tree with the greatest range in North America, stretching from Alaska through all of Canada to Newfoundland and southwards to Virginia, and in the Rocky Mountains as far south as northern Mexico. Between them, these two closely-related aspen species cover virtually the entire boreal biome. Because they are so similar, ‘aspen’ will be used in this article as a generic term for both species, except where it is specified otherwise.
It is not only the range of aspen that is remarkable, though, as it has a number of other unusual characteristics which have been drawing increasing attention from scientists, researchers and conservationists in recent years. As a fast-growing pioneer species, aspen regenerates profusely after disturbance such as fire, and often occurs as dense stands of even-aged trees. This regeneration takes place almost entirely by vegetative reproduction, as aspen rarely propagates from seeds. Instead, new shoots, or ramets, grow from the roots of a parent tree, and these stay connected underground, even once the shoots have matured into trees. All the interconnected trees are a single organism, known as a clone, which exhibits synchronous behaviour – for example, all the component trees will come into leaf at the same time. Because aspen is dioecious, an individual clone is either male or female, and research on Populus tremuloides in the USA has revealed how large individual clones can be. One clone in Utah, nicknamed ‘Pando’ (from the Latin for ‘I spread’), contains over 47,000 individual stems and covers an area of 43 hectares – with an estimated weight of over 6,000 tonnes, this is the world’s largest known organism.
The clonal reproductive strategy of aspen also means that it is extremely long-lived. Although individual stems may only survive for a maximum of 200 years, the clone itself lives for much longer, as new stems grow to replace those which die. Some clones of Populus tremuloides in the USA have been estimated to be at least 8,000 years old, making them possibly the oldest organisms on the planet. It has even been speculated that aspen is ‘theoretically immortal’, and some researchers have suggested that clones may reach an age of a million years or more, based on the resemblance of the leaves on aspen trees today to fossilised ones!
Another interesting feature of aspen’s clonal reproduction method is that the roots of a tree can survive underground after the death of the trunks above ground. The roots will continue to produce new ramets, and they in turn provide enough nutrients through photosynthesis to keep the roots alive until some ramets can grow successfully into new trees. A further feature which helps aspen in its growth is its ability to absorb the sun’s energy through its trunk – the greenish tinge often seen on aspen trunks indicates the presence of chlorophyll there, which carries out the photosynthesis.
In Europe, recent research has highlighted the ecological importance of Populus tremula for a wide range of forest species, from mosses and lichens to fungi and insects. Notable species associated with aspen include the aspen bracket fungus (Phellinus tremulae) which is pathogenic and therefore a significant cause of mortality for the tree; aspen brittle-moss (Orthotrichum gymnostomum); and the dark bordered-beauty moth (Epione vespertaria). There is also a unique community of saproxylic insects (ie insects which depend on dead wood) associated with dead aspen trees, many of which are rare in Europe, and in 1997 researchers studying this community in Scotland discovered a previously-unknown species of fly (Ectaetia christiei). Aspen is also drawing attention in Scotland in the light of the proposed reintroduction of European beavers scheduled for 2003. Aspen is a key winter food for this aquatic rodent, which was extirpated from the UK in the 16th century, as a result of hunting for its fur.
Trees for Life has been working to protect and restore aspen in the Highlands of Scotland since 1991, and we are currently seeking funds to expand this programme significantly, with a full-time project officer dedicated to it for the next 3 years. Our work includes the surveying of existing aspen sites (212 mapped to date), propagation of aspens from root sections (7,500 grown by 2002), the protection of regenerating ramets at existing sites and a research programme, in cooperation with Edinburgh University, into the ecology of aspen. The next steps are focussed on restoring the habitat for the aspen-dependent species of flora and fauna, through expanding and linking up existing aspen stands and creating new ones in appropriate locations. In doing so, we aim to produce and implement an aspen recovery plan which will provide a viable future for aspen and all its associated species in the northern Highlands.
In conclusion, aspen is a charismatic species displaying a number of spectacular characteristics, and it also supports a unique assemblage of other organisms. Occurring throughout the boreal zone, it can be seen as a unifying symbol of the northern forests and can act as a flagship species for their conservation.
Alan Watson Featherstone
(This article featured in the publication Taiga News in 2003)
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